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NetBIOS Session Service
Port 139 is primarily used by the NetBIOS Session Service, enabling communication between devices on a local network using the NetBIOS protocol. It facilitates file and printer sharing over Windows networks by allowing sessions over TCP/IP. Historically integral to Windows networking, it allows systems to identify each other and establish resource-sharing connections..
Port 139 corresponds to the NetBIOS Session Service, which establishes and maintains sessions for communication over NetBIOS on TCP/IP networks. This port is a vital part of the legacy SMB (Server Message Block) protocol stack, specifically SMB over NetBIOS framing. It allows devices within the same local segment to exchange session-based data, performing operations such as file and printer sharing and inter-process communications.
When a connection is initiated on port 139, the NetBIOS Session Service facilitates session establishment, maintenance, and termination. It transmits session-control packets to manage the communication between hosts. Name resolution and datagram distribution are handled by related NetBIOS services on other ports (like 137 and 138), whereas port 139 focuses solely on session-level data exchange.
Modern implementations have moved towards using port 445 (Direct hosted SMB) that bypasses legacy NetBIOS framing for efficiency and security. Nevertheless, port 139 remains operational in many environments for backward compatibility, especially where mixed or older Windows systems are present.